Part of being an avid sports fan is a propensity for making predictions. We do it all the time, as in:

“Here comes Buster Posey. I bet he’s going to crush one over the fence.”

Well, two lifelong Giants fans have launched an iOS app that allows fans to challenge one another’s predictions in real-time, whether on the couch, in the bar, or out at the ballpark.

GrabFan, which debuted at the end of March, is the brainchild of Steve Smith and Cassidy Lavin, who grew up in Orinda and are long-time fantasy sports enthusiasts.

GrabFan is sports game where players earn “Fandom” through answering trivia questions about their favorite teams or by issuing challenges to other fans with a virtual currency called “GrabBucks.”

These are redeemable at participating bars (Bullitt on Polk Street is an early partner), or for merchandise, including tickets to Giants' games.

Probably the most popular feature of this game over time will be the customizable challenges in real-time. You can issue these to the general community playing the game or just to your friends.

“You can set a maximum number of people for each challenge if you wish,” explains Smith. “The flexibility is all with the user. When watching a game in the third inning, you can be making predictions along with your friends and other fans.”

The game utilizes official statistics licensed from Stats, Inc., which also supplies all the big sports services like ESPN, Yahoo, Fox, CBS, etc.

Lavin says that early usage indicates people tend to issue open challenges “right before game time, and then, during the game there are more fan-to-fan challenges.”

Although baseball is the main focus for GrabFan at the moment, the team plans to include all major sports going forward. They also are developing a merchant app for stores and bars to connect with fans, as well as an Android version of the game, which should be ready by the end of this month.

You can download the free iOS app and sign in with an email account, although, Smith says, “We recommend you sign in with Facebook because then you have people (in your social graph) to play with right at the start.”

The user base has been growing via word-of-mouth to date but the company is about to kick off a more serious marketing effort, including a car painted with the faces of Giants players that will probably be seen around the ballpark area soon.